The invention concerns a method and a system for real-time navigation using satellite-transmitted three-carrier radio signals and ionospheric corrections, more precisely corrections obtained by means of a continuously updated real-time ionospheric model, the model being based on data from a satellite navigation system, for example implemented like a three-dimensional voxel model.
It is particularly though not exclusively, applicable to the field of high-precision instantaneous navigation, typically having a precision within one decimeter, as will be shown below, at distances on the order of hundreds of kilometers or more.
By way of illustration, we will hereinafter focus on the preferred application of the invention, without in any way limiting its scope.
One of the current commonly used techniques for obtaining a precise positioning of an “object,” whether stationary or moving, and in the latter case, data relative to its position, its movement, the direction of this movement and/or its speed, is to use radio signals transmitted by artificial satellites orbiting around the earth. The term “object” should be understood in its most general sense, notably a land, sea or air vehicle. For simplicity's sake, we will hereinafter refer to this “object” as a “rover.”
There are various known techniques for obtaining the aforementioned positioning. They are based in particular on the knowledge of the instantaneous position of several satellites in space (or a constellation of satellites, as these satellites may or may not be geostationary) and of the propagation speed of the radio waves. High-precision clocks are installed aboard the satellites, and the transmitted signals include time-stamped information, which makes it possible to know precisely the moment of both transmission and reception. It is thus possible to determine the theoretical distance separating a satellite in view of the rover from the latter at a given moment, knowing the propagation speed of the waves and the time they take to reach the rover. If a sufficient number of satellites is observable, it is thus possible to determine the coordinates of the rover relative to a reference frame, in two dimensions (longitude and latitude on the earth) or even three dimensions (longitude, latitude and altitude/vertical).
However, as will be shown, because of the accumulation of errors in the measurements due to various causes, the distances calculated are only approximate, and the determination of the position of the rover suffers from a more or less substantial degree of imprecision, depending on the technologies used.
One of the best known satellite navigation systems is the system known as “GPS” for “Global Positioning System.”
Customarily, “GPS” (or similar system) satellites transmit in two frequency bands, generally designated L1 (a carrier frequency equal to 1.575 GHz) and L2 (a carrier frequency equal to 1.227 GHz), hence the adjective “dual frequency” that is applied to them.
The use of these two frequencies, in accordance with certain methods well known to one skilled in the art, makes it possible to improve the precision of the determination of a rover's position relative to a reference frame, but it requires “GPS” receivers that are more complex and more costly.
A position determination can be made using two main methods: real-time, or after-the-fact, by performing what is known as a “post-processing.” The first case is commonly referred to as “single-epoch” resolution (a term that will be used below) or instantaneous resolution, the calculations being performed during a single observation “epoch.” The second method (“post-processing”) makes it possible to improve precision. However, while the latter method does not present any major drawbacks for slow-moving rovers (ships, for example), it is not appropriate for rovers that move very fast (aircraft, for example).
Precision can be further improved by combining the signals transmitted by the satellites with signals originating from fixed ground reference stations whose positions are well known. However, if the rover travels great distances, it is necessary for this network of stations to be relatively dense, especially in cases where high precision is desirable in the determination of the rover's position, which correspondingly increases the cost of the global system.
Moreover, among the numerous causes of errors, differential ionospheric refraction, when considering distances equal to tens of kilometers or more, is one of the main problems affecting capacities for instantaneous resolution of carrier phase ambiguity, and consequently the capability to provide a navigation wherein the precision is on the order of one centimeter with dual-frequency global navigation satellite systems such as the aforementioned “GPS” system. This characteristic will remain true with respect to future three-frequency systems like the “GALILEO” system and the “Modernized GPS” system.
In essence, the three-carrier systems currently being planned offer the potential advantages of a high success rate and high integrity in instantaneous ambiguity resolution, with a minimal number of geodetic calculations. This is particularly due to the fact that a higher quantity of different data (i.e., associated with the aforementioned three frequencies) is made available, which correspondingly improves the chances of obtaining an instantaneous (“single epoch”) ambiguity resolution.
But here again, this resolution can be seriously affected by ionospheric refraction, as explained below.
In order to achieve high precision in the instantaneous determination of the position of a rover, particularly a rover that travels great distances, there is still a need to implement techniques that make it possible, in particular, to reduce the harmful influence of ionospheric refraction.
In the prior art, various methods for meeting this need have been proposed.
For example, there is the method known as “TCAR,” for “three-carrier ambiguity resolution.” This method is described in the article entitled “ANALYSIS OF THREE-CARRIER AMBIGUITY RESOLUTION (TCAR) TECHNIQUE FOR PRECISE RELATIVE POSITIONING IN GNSS-2,” by U. VOLLATH et al., published in “Proceedings of the ION GPS” 1998, IX-O-13, pages 1–6.
There is also the method known as “CIR,” for “cascade integer resolution.” This method is described in the article by Jaewo JUNG et al., entitled “Optimization of Cascade Integer Resolution with Three Civil GPS Frequencies,” published in “Proceedings of the ION GPS 2000.”
These two techniques share a similar basic approach: the double difference ambiguities of integers are successively resolved by calculating wave frequency beats. This calculation is performed from the longest to the shortest beat wavelength, including combinations of so-called “wide” lane and “extra wide” lane carrier phases (with wavelengths of 7.480 m and 0.862 m, respectively), and a first carrier at the “L1 frequency” (with a wavelength of 0.190 m).
The “TCAR” method in particular constitutes a simple approach that tries to resolve the full set of ambiguities instantaneously (in “single epoch” mode). But the performance of “TCAR” is strongly affected by the ionospheric refraction decorrelation that occurs with distance. In fact, as explained below, ionospheric delay is a problem when (as in the case of two-frequency systems) the value of its double differential is more than 0.26 TECU (which corresponds to a 4-cm delay for L1).
A “TECU” is a unit used to describe certain electrical characteristics of the ionosphere. In essence, the ionosphere can be described using a map that represents a count of the total number of electrons, or “TEC” (for “Total Electron Content”). The map represents the integration of the number of electrons in a vertical direction as a function of latitude and longitude. A unit of TEC is referred to as a “TECU” (for “TEC Unit”), with one TECU=1016 electrons contained in a cylinder aligned on the line of observation of an observed satellite, the cross-section of which is 1 m2. The charged particles in the ionosphere are generated by the sun, whose radiation intensity varies naturally as a function of the time in question. Because the earth rotates on its axis underneath the ionospheric layer, the “TEC” map is normally considered to represent a reference frame that is fixed relative to the sun, but that changes as a function of time.
The above-mentioned threshold is easily exceeded, as may be seen by consulting ionospheric (“TEC”) maps of vertical delays calculated from “GPS” data. Such maps are issued, for example, by the “Jet Propulsion Laboratory,” the University of Bern, etc., and published on the Internet by the “University Corporation for Atmospheric Research” and other similar organizations.
Consequently, in order to further improve the “TCAR” method, an integrated approach known as “ITCAR” (for “Integrated TCAR”) was developed. This technique is described, for example, in the above-mentioned article by VOLLATH et al.
This technique uses search algorithms and a navigation filter wherein the ambiguities are part of the output signals and the residual ionospheric errors are roughly estimated. For a more detailed description of the techniques used, it would be worthwhile to refer to this article.
However, although it provides a significant improvement, the “ITCAR” technique is nonetheless still affected by the lack of knowledge of the double difference of the ionospheric refraction, thus limiting the success rate of the ambiguity resolution for distances greater than several tens of kilometers, as described in the article by VOLLATH et al., entitled “Ambiguity Resolution Using Three Carriers—Performance Analyzing Using ‘Real’ Data, published in “GNSS Symposium,” Seville, May 2001.
It has also been proposed, again with a view to improving the precision of the determination of a rover's position relative to a reference frame, to combine a real-time ionospheric model of the ionosphere, obtained from “dual-frequency” data generated by a network of fixed stations, with data from a geodetic program, and to use such data to perform ionospheric corrections. This method has been used with some success in resolving ambiguities in real-time in two-frequency systems of the “GPS” type.
One method of this type, called “WARTK” (for “Wide Area Real-Time Kinematics”), is described for example in the article by Hernández-Pajares et al. entitled “Tomographic Modeling of GNSS Ionospheric Corrections: Assessment and Real-Time Applications,” published in “ION GPS” 19–22 Sep. 2000, pages 616–625. This method does make it possible to attenuate the harmful effects of the disturbances experienced by the radio waves propagating in the ionosphere, and consequently, to substantially improve the success rate of the phase ambiguity resolution and the determination of a rover's position relative to a reference frame, but it requires a large number of calculations to produce the aforementioned model in real time and to determine, also in real time, the ionospheric corrections to be applied to the distance measurements.